Ah, a fun thought exercise. As always, if you want professional tax advice, talk to a local tax professional.
That being said, there are multiple potential issues. Like some of the others have touched on, will she be an W-2 employee, or will she be an independent contractor, which in turn come down to actual rights and responsibilities. Walkwalkwalk has given you a decent link about it. Though I've definitely heard of people doing contract research for law firms.
If she is an employee, she most likely than not will create nexus in Florida for the law firm. As that will potentially cause the law firm to be subject to income tax reporting (depending on the law firm entity), sales tax reporting, employment taxes, franchise tax (if FL has one), etc. So, this will more likely than increase the law firms tax filing burden. How difficult the employment tax situation (for the law firms' payroll processor) still depend on what taxes the firm is subject to and if they do payroll in house or through a 3rd party (such as ADP).
As for your sister. If she is being treated as an IC, she will be responsible for both the EE and ER portion of SE taxes, benefits, etc. And depending on if she travels back to NC and SC to do work, after moving to FL, she may have income tax filing requirements in multiple states (both as an IC and a W-2 employee). I'm not aware of any special rules about telemuting employees in FL, NC, SC (as this is an emerging area of tax law and different states treat things differently), but you would want to look to be sure of what her state filing requirements would be.
